The present invention relates to a process for laser-assisted liquid phase etching, and particularly to an improved etchant solution for use in such etching process.
Laser-assisted etching is a technique of increasing interest and application in the area of microelectronics, especially in circuit repair and maskless customization of generic circuit interconnects. Laser-assisted, liquid phase etching is a process whereby a narrow laser beam acts to heat a workpiece surface in the presence of an etching solution, and thereby selectively enhances the chemical-etchability thereof. The activity is restricted to the region of the workpiece where the beam is incident. Typically, the process is focused on the removal of select segments of the underlying wiring ("conductors") of the circuit chip or package.
The advent of high-density inter-chip interconnect technologies presents new problems for the application of laser etch methods. The fabrication of monolithic multi-chip substrates (MCS's) for integrated circuits is a sufficiently complex process in which some amount of repair of defective interconnects is necessary for economic manufacture. For example, it may be necessary to locally etch completely through the thickness of a conductor at a particular point along its length, in order to electrically disconnect two portions of the original circuit configuration. Such a need may arise if one conductor is inadvertently connected, or "shorted", to an adjacent conductor. It is also desirable to rewire connections in order to implement design changes in the substrate, after it has been fabricated and tested. This may also be preferentially performed by localized etching.
The demanding requirements of localization (8-30 micron linewidths) for repairing conductor lines suggest that localizable energy sources are appropriate tools to initiate and control repair process technologies. Laser beams are preferable to particle beams, such as electron or focused ion beams, by virtue of the much larger particle fluxes (greater beam-assisted process rate) which can be readily achieved.
The advantages of laser-assisted chemical etching (including both gas and liquid-phase etching) has already been demonstrated. Laser-assisted chemical etching to date has been concerned with conductors made from aluminum, titanium/gold, molybdenum, polycrystalline silicon and tungsten, among others. For example, Tuckerman (D. B. Tuckerman, IEEE Elec. Dev. Letter, EDL-8,540 (1987)) described the utility of laser gas-phase etching in forming chip-to-substrate interconnects for gold-on-silicon dioxide substrates. Also, Tsao and Ehrlich (J. Y. Tsao and D. J. Ehrlich, Appl. Phys. Letter 43, 46 (1983)) and J. Y. Tsao and D. J. Ehrlich, J. Electrochem Soc. 133, 2244 (1986)) investigated liquid phase etching of 1-.mu.m thick aluminum lines in a mixture of nitric and phosphoric acids, as might be appropriate for photomasks and on-chip interconnects. However, this latter etch chemistry and process is highly specific to aluminum of refractory oxide substrates, and is not necessarily applicable to other metals and dielectric systems.
Furthermore, the application intended for this aluminum etch is the repair of quartz photomasks for which the aluminum conductors are only a few hundred nanometers thick. Since the optimal laser irradiation conditions depend significantly on the thickness and width of the conductors to be etched, as well as the substrate material, an entirely different set of conditions are required to permit the successful etch of different conductors of different dimensions.
A particularly preferred material combination utilized in the fabrication of MCS's, particularly medium-film MCS's having film thicknesses and widths greater than about one micron but less than about 30 microns, is copper on polyimide. Copper conductors have low resistivity and are comparatively inexpensive to fabricate. Polyimide has a comparatively low dielectric constant and can be formed into thick, planarizable layers which are necessary to achieve a low cross-talk, controlled-impedance interconnect. The thermal properties of copper and polyimide make laser etching the preferred means of a cutting copper on polyimide. In particular, the fact that the melting point of copper (1080.degree. C.) is much higher than the decomposition temperature of the polyimide on which it is situated (400.degree.-500.degree. C.) rules out thermal ablation - it is unlikely that the conductors could be cut by melting the copper, without severely damaging the underlying polyimide.
The presently known laser-assisted chemical etching techniques, which are highly material and process specific, are inapplicable for etching copper/polyimide interconnect substrates. Accordingly, there exists a need for a laser-assisted, liquid-phase etching process for etching copper conductors patterned onto polyimide layers.